fix(gottext): write a bare calendar day with only the parts it is handed

gottext/CHANGELOG.md

@@ -11,6 +11,7 @@ from CI.
 
 - `mismatched` reads the tags a message carries, so a translation that adds, drops, renames or repeats one is named.
 - `formatTime` writes a bare wall clock time, such as `09:30`, as the time it names for every reader, instead of Invalid Date.
+- `formatDate` writes a bare calendar day with only the parts it is handed, and keeps its year in a calendar of its own.
 
 ## [0.8.0] - 2026-11-02
 

gottext/src/format.ts

@@ -17,6 +17,29 @@ const DAY: Intl.DateTimeFormatOptions = { day: '2-digit', month: '2-digit', year
 /** CLOCK is how a time is written. */
 const CLOCK: Intl.DateTimeFormatOptions = { hour: '2-digit', minute: '2-digit' }
 
+/** DATE_OPTIONS names every option a date format reads, other than its time zone. */
+const DATE_OPTIONS = [
+	'calendar',
+	'dateStyle',
+	'day',
+	'dayPeriod',
+	'era',
+	'formatMatcher',
+	'fractionalSecondDigits',
+	'hour',
+	'hour12',
+	'hourCycle',
+	'localeMatcher',
+	'minute',
+	'month',
+	'numberingSystem',
+	'second',
+	'timeStyle',
+	'timeZoneName',
+	'weekday',
+	'year',
+] as const satisfies readonly (keyof Intl.DateTimeFormatOptions)[]
+
 /**
  * Stores the locale dates, times, numbers and money are written in.
  * @param locale - The locale to write in, or undefined to forget it.
@@ -45,18 +68,17 @@ function writeDay(at: Date | string, locale: string, options: Intl.DateTimeForma
 		return ''
 	}
 	const day = typeof at === 'string' && CALENDAR_DAY.test(at)
-	return new Date(at).toLocaleDateString(locale, day ? inUTC(locale, options) : options)
+	return new Date(at).toLocaleDateString(locale, day ? inUTC(options) : options)
 }
 
 /**
- * Returns the options a caller handed as a date format resolves them, showing UTC.
- * @param locale - The locale the date is written in.
+ * Returns the date options a caller handed, showing UTC.
  * @param options - The options the caller handed.
  * @returns The options a bare calendar day is shown with.
  */
-function inUTC(locale: string, options: Intl.DateTimeFormatOptions): Intl.DateTimeFormatOptions {
-	const resolved = new Intl.DateTimeFormat(locale, options).resolvedOptions()
-	return { ...resolved, timeZone: 'UTC' } as Intl.DateTimeFormatOptions
+function inUTC(options: Intl.DateTimeFormatOptions): Intl.DateTimeFormatOptions {
+	const handed = Object.fromEntries(DATE_OPTIONS.map((name) => [name, options[name]]))
+	return { ...handed, timeZone: 'UTC' } as Intl.DateTimeFormatOptions
 }
 
 /**

gottext/test/format.test.ts

@@ -54,6 +54,51 @@ test('writes a date with the options it is handed in the format locale', () => {
 	expect(formatDate(new Date(2026, 6, 30), { month: 'long' })).toBe('julio')
 })
 
+test('writes a bare calendar day with only the parts it is handed in the format locale', () => {
+	rememberFormatLocale('es-ES')
+	vi.stubEnv('TZ', 'UTC')
+
+	expect(formatDate('2026-09-30', { month: 'long', year: 'numeric' })).toBe('septiembre de 2026')
+	expect(formatDate('2026-09-30', { month: 'short', year: 'numeric' })).toBe(
+		formatDate('2026-09-30T12:00:00Z', { month: 'short', year: 'numeric' }),
+	)
+})
+
+test('keeps the year of a bare calendar day in a calendar of its own', () => {
+	rememberFormatLocale('es-ES-u-ca-chinese')
+	vi.stubEnv('TZ', 'UTC')
+
+	expect(formatDate('2026-08-20')).toContain('2026')
+	expect(formatDate('2026-08-20')).toBe(formatDate('2026-08-20T12:00:00Z'))
+})
+
+test.each<Intl.DateTimeFormatOptions>([
+	{ calendar: 'chinese' },
+	{ numberingSystem: 'arab' },
+	{ era: 'long', year: 'numeric' },
+	{ weekday: 'long', day: 'numeric' },
+	{ day: 'numeric', hour: 'numeric' },
+	{ day: 'numeric', hour: 'numeric', minute: '2-digit' },
+	{ day: 'numeric', hour: 'numeric', minute: '2-digit', second: '2-digit' },
+	{ day: 'numeric', second: '2-digit', fractionalSecondDigits: 3 },
+	{ day: 'numeric', hour: 'numeric', dayPeriod: 'long' },
+	{ day: 'numeric', hour: 'numeric', hour12: false },
+	{ day: 'numeric', hour: 'numeric', hourCycle: 'h23' },
+	{ day: 'numeric', timeZoneName: 'short' },
+])('writes a bare calendar day handed %o as midnight UTC on that day', (options) => {
+	rememberFormatLocale('en-US')
+	vi.stubEnv('TZ', 'UTC')
+
+	expect(formatDate('2026-09-01', options)).toBe(formatDate('2026-09-01T00:00:00Z', options))
+})
+
+test('refuses a time style on a bare calendar day as on a timestamp', () => {
+	rememberFormatLocale('en-US')
+
+	expect(() => formatDate('2026-09-01', { timeStyle: 'short' })).toThrow(TypeError)
+	expect(() => formatDate('2026-09-01T00:00:00Z', { timeStyle: 'short' })).toThrow(TypeError)
+})
+
 test('writes a time as two digit hours and minutes on the es-ES clock', () => {
 	rememberFormatLocale('es-ES')
 	vi.stubEnv('TZ', 'UTC')